Conventionally, it is known that the revolutional speed of a wheel of a vehicle such as an automobile is detected, and after that, running speed thereof is obtained for determining the running speed of the vehicle.
However, the method is affected by alterations in the diameter of the wheel, and moreover when the wheel is slipped, speed measuring thereof is not possible. Accordingly, the measured speed can not be used for detecting an exact speed which is necessary for an antiskid control.
Therefore, apparatus, which is able to detect an absolute velocity of a vehicle by using a so-called a spatial filter, has been developed, so that the spatial filter is used for an antiskid control. For instance such a speed measuring apparatus using a spatial filter is disclosed in FIGS. 2 and 3 of Japanese Patent Laid-Open No. 52-143081 published on Nov. 29, 1977 in the title of "Speed Measuring Apparatus of running object using Spatial Filter type Detector".
In a speed measuring apparatus using such a spatial filter, it is necessary that an image formation optical system be used for forming an optical image of a surface of a measuring object, such as a road, on a photoelectric transformation surface of the spatial filter. On account of this, when the distance between a measuring apparatus comprising a spatial filter and a measuring object is changed, measuring error increases.
For preventing the error, the prior art apparatus explained above art discloses a so-called telecentric optical system which equips a light shield plate having a small hole between an objective lens in an image formation optical system for forming an image of a surface of a measuring object and an photoelectric transformation surface of the spatial filter. According to the prior art explained above, the change of magnification of an optical image caused by the change of the distance is prevented and measuring error does not occur.
Hereunder, we will explain the prior art. Referring to the prior art shown in FIG. 2, an objective lens 1 is opposed to a surface of a measuring object 10 such as a road surface which is in the opposite side of a vehicle such as a running automobile. The surface image of the measuring object 10 has random patterns of irregularities or density of the surface which are irradiated by the light 41 from a light source 9, and are projected on a spatial filter detector 2. As the detector 2 of the spatial filter, for instance as shown in FIG. 3, a silicon solar cell having light receiving portions 12 formed in the shape of teeth of a comb is used. In FIG. 3, 13 denotes electrodes, 14 substrate, and 15-1, 15-2, 16 lead wires.
Movement of a surface image pattern having the same pitch as the pitch P of the light receiving portion 12 emphasizes the surface images of the measuring object 10. The other patterns of the surface image of the measuring object 10 not having the same pitch as the pitch P of the light receiving portion 12 are not output, since the change of the photo currents from each light receiving portion 12 of the silicon solar cell 2 eliminate each other and the output signal of the detector of the spatial filter system, which is the summation of each photo current, becomes almost independent of time.
The frequency output from the detector 2 corresponds to a reciprocal of a time which is equal to a time interval in which one pitch of the surface image of the measuring object 10 passes same length of one pitch P of the light receiving portion 12 of the detector 2 of the spatial filter system. When the running speed of the vehicle is v, the arranged pitch of the light receiving portion 12 of the detector 2 is p, and the magnification of the projected image is M, the frequency f of the signal obtained at a frequency detecting circuit 5 from the detector 2 through an amplifier 3 is ##EQU1## and proportional to the velocity v, so that the velocity can be detected.
Incidentally, when the objective lens 1 is used as an image formation optical system, and the distance from the measuring object 10 to the lens 1 is d as shown in FIG. 2, but a light shield plate 11 having a small hole on a focus is not disposed; the magnification M of the projected image becomes ##EQU2## In this case, when the distance d is changed, the output frequency f of the detector 2 of the spatial filter system is varied so that an error in speed detection is caused.
On the contrary, when the light shield plate 11 having a small hole on a focus of the lens 1 is arranged as disclosed on FIG. 2, in the light reflected from one point P on the surface of the measuring object 10 only light 45 transmitted on a parallel direction of the optical axis of the lens 1 reaches the detector 2 of the spatial filter system. Even if the distance d is changed, a light reflected from the point P passes along the same route and reaches the same position on a light receiving element 32 arranged on a substrate 31 of the detector 2 of the spatial filter system. Accordingly, in the apparatus of FIG. 2, the magnification M of the projected image does not change corresponding to the change of the distance d. However, the apparatus shown in FIG. 2 can not avoid the amplitude of the output signal becoming smaller, since the brightness of the image on the detector 2 of the spatial filter system 2 becomes dark.
As explained above, the apparatus of the prior art does not consider the decrease of the light amount based on using the light shield plate, and the decrease of the signal level from the spatial filter, so that it has a drawback of measuring error depending on decreasing the signal versus noise (S/N).
A spatial filter used for an automobile has to be located at the outside of the floor surface of the automobile facing to the ground. In this case, the spatial filter has difficulty obtaining sufficient reflected light from the measuring object, since the objective lens is easily soiled. Accordingly, the spatial filter of the prior art is in the condition that the S/N ratio is easily lowered, so that there is a danger that a large measuring error might occur or that measurement is not possible.